Oxidation of substrates, such as semiconductive substrates, has been performed thermally in either wet or dry ambients.
When the goals of ultra-microminaturization begin to come in reach, conventional processes of thermal oxidation become increasingly untenable because at the elevated temperatures that are involved in the developing complex structures, the device impurity profiles become redistributed and this, in turn, impacts device designs significantly.
An effort has been underway to find an oxidizing technique that can be practiced at lower temperatures, preferably below 700.degree. C., at which temperature impurity diffusion coefficients become very small.
The art of plasma formation of oxide on semiconductors has been referred to as plasma growth, as plasma oxidation, or under circumstances where the semiconductor substrate is part of the anode electrode, the process is called anodization. In the article in "Solid State Electronics", Volume 17, page 627, 1974, oxide formation at constant voltage bias with no control of temperature is recorded.
Plasma oxidation of conductive or semiconductive materials and especially of a semiconductor, such as silicon, is of great interest since it has the potential of providing low temperature processing coupled with high oxidation rates. Low oxidation temperature essentially eliminates impurity redistribution and defect formation effects that are associated with normal high temperature oxidation processes.
The reported results on plasma oxidation vary widely with respect to the kinetics of the oxidation (e.g. linear or linear-parabolic), the frequency of the applied field (e.g. RF or microwave), the mode of plasma generation, such as with or without electrodes in the system, the use of biased or unbiased substrates (i.e. plasma anodization or oxidation), and the properties of the resultant plasma grown oxide (high flat band voltage). However, in all of these cases, the area of uniformly oxidized silicon was found to be very small and usually less than or equal to two square centimeters. In addition, the temperature of the oxidation was determined by the extent of plasma heating only. Accordingly, the reported results have not, in fact, been very promising. In our copending U.S. patent application, Ser. No. 16,648, filed Mar. 1, 1979, now Pat. No. 4,232,057 (427/39) plasma oxide formation on the semiconductor substrates is disclosed wherein the temperature control of the substrate, the pressure control, and the plasma power control are independent of each other. In the preferred embodiments in said application, the pressure is less than 3 mtorr. When a semiconductive substrate, such as silicon, is placed perpendicular to the direction of gas flow at varying distances from the plasma generating region in low pressure regions, such as at about 3 mtorr and less, SiO.sub.2 is formed on the surface of the wafer facing the plasma. Upon examining the Si-Si0.sub.2 interphase using conventional SEM techniques, it was found that SiO.sub.2 is a deposited film. The thickness of the film increases linearly with time. In addition, it was noted that the rate of deposition increases with increasing power, decreasing pressure and decreasing distance from the plasma generating region. In addition, it was found that the rate of deposition is independent of the temperature of the substrate. Moreover, in this method, the depositied oxides exhibited etch rates and refractive indices quite comparable to thermal oxide. Fixed charge and interface state densities were reasonable but on the high side following high and low temperature pre- and post-metallization annealing processes respectively.